using System;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Please enter a number that you would like the factorial of.");

        start:

        string x = Console.ReadLine();

        ulong num;

        ulong y = 1;

        if (ulong.TryParse(x, out num))

        {

            if (num <= 65)

            {

                for (ulong i = 1; i <= num; i++)

                {

                    y *= i;

                }

​

                Console.WriteLine("The factorial of " + num + " is " + y + ".");

                Console.WriteLine("Do you want to enter a new number? y/n");

                string ans = Console.ReadLine();

                if (ans == "y" || ans == "Y")

                {

                    goto start;

                }

            }

            else if (num > 65)

            {

                Console.WriteLine("Please enter a smaller number.");

                goto start;

            }

        }

        else

        {

            Console.WriteLine("Please enter a positive whole number.");

            goto start;

        }

    }

}